1. Field of the Invention
The present invention relates to a flip-chip technology; and, more particularly, to sa flip-chip bonding structure incorporating therein an MCM-D (Multi Chip Module-Deposited) substrate.
2. Background of the Related Art
As a consequence of the recent rapid increase in demand for more band-width, mm-wave applications such as a short-range broad-band wireless communication are currently attracting a great interest and a investment. In order, however, for these applications to be commercially viable, a commercial package technology capable of imparting compact and high-performance mm-wave modules with low manufacturing cost is a must. MCM-D technology is one of the best candidates for realizing this objective, because, as well as allowing system-on-package (SOP) approach as it is based on thin-film dielectric layers, it is capable of providing high resolution patterns, an absolute necessity for accommodating mm-wave frequency having very short wavelength. In addition, the flip-chip interconnection has a number of advantages over the wire-bonding interconnection. To mention a few, they include: (1) lower parasitic elements due to the shorter interconnection length; (2) lower assembly cost; and (3) higher reproducibility. Accordingly, by mounting active components such as a monolithic mm-wave integrated circuit (MMIC) or an active device on the MCM-D substrate by means of the flip-chip technology, the objective described above can be achieved.
There are some technical issues which must resolved, however, if the flip-chip technology is to be used as the interconnection between the active components and the MCM-D substrate in mm-wave applications, the issues, mainly relating to the inherent properties of the MCM-D substrate, such as the CTE (Coefficient of the Thermal Expansion), the thermal conductivity, the dielectric constant, and the suppression of package-related parasitic modes, all of which are critical to the over-all performance of the module. The above mentioned properties are critical in that: (1) the CTE of the MCM-D substrate has to be similar to that of the active components mounted on the MCM-D substrate to improve the thermo-mechanical reliability of the flip-chip structure; (2) the MCM-D substrate must have a high thermal conductivity to effectively dissipate the heat generated by the active components; (3) the MCM-D substrate should have a low dielectric constant to reduce the proximity effect between the active components and the MCM-D substrate; and (4) the MCM-D substrate must have a lossy property or support shorting via holes to suppress package-related parasitic modes caused by conducting backside.
Currently, the most widely used flip-chip substrates are based on alumina. The alumina substrate is an insulator substrate, and could be used to make a transmission line having good transmitting characteristics. Also, the CTE of alumina substrate, 6 ppm/° C., similar to that of the chip components, 7 ppm/° C., ensures a reliable flip-chip structure.
However, the high dielectric constant of the alumina substrate, about 9.8, can result in high proximity effects between the active components and the alumina substrate, and the relatively low thermal conductivity of the alumina substrate, 30 W/(m·K), in a poor dissipation of the heat generated by the active components.
To suppress the package-related parasitic modes, silicon substrates have been used as a substitute for the alumina substrates. However, the silicon substrate is saddled with a poor transmissibility. Referring to FIG. 1, there is shown a prior art flip-chip bonding structure including a Si-substrate 101, a dielectric layer 102, a transmission line 103, a flip-chip bump 104 and an active component 105. In the prior art, to solve the above-described shortcoming of the silicon substrate 101, a dielectric layer 102 having a low dielectric loss and a low dielectric constant, such as BCB (BenzoCyloButene), is coated on the entire surface thereof, making it possible to form a transmission line 103 with good transmitting characteristics. Further, the low dielectric constant of the dielectric layer 102 reduces the proximity effects between the silicon substrate 101 and the active component 105. However, in case of BCB, a relatively high CTE thereof, around 56 ppm/° C., compared to that of the active component 105, causes a mismatch of the CTEs between the silicon substrate 101 and the active component 105, which, in turn, causes cracks to form in the flip-chip bonding bump 104, thereby lowering the reliability of the flip-chip bonding structure. Also, the low thermal conductivity of BCB, around 0.02 W/(m·K), results in a poor dissipation of the heat generated by the active component.